Field of the Invention
The present invention relates to rotary furnaces for the production of hydraulic binders such as cement and lime.
Related Art
Such rotary furnaces have a generally substantially cylindrical shape, the length of the cylinder being much greater than its width. The furnace rotates around a rotation axis which is inclined with respect to the horizontal and corresponds to the longitudinal axis of the cylinder. The material to be pyroprocessed in the furnace travels downwards through the furnace under the effect of gravity. The furnace comprises a burner assembly at its lower end for the combustion of main fuel with combustion oxidizer so as to generate the heat necessary for pyroprocessing. The flame generated by the burner assembly is directed substantially along the longitudinal direction of the furnace. The flue gases generated in the furnace are evacuated from the furnace at its upper end.
The pyroprocessed material, such as lime or clinker, is transferred from the furnace to an air-cooled material cooler.
In order to reutilize the thermal energy of the hot cooling air leaving the cooler, it is known to use the hot cooling air as a secondary oxidizer for the combustion of the main fuel. In that case, the burner assembly injects the main fuel and primary combustion oxidizer into the furnace so as to generate partial combustion of the main fuel with the primary combustion oxidizer. Hot air from the material cooler is fed to the furnace to provide secondary combustion oxidizer for the substantially complete combustion of said main fuel.
It is a problem with rotary furnaces for the production of hydraulic binders that thick localized deposits or build-ups, also referred to as rings, form on the cylindrical wall of the furnace during furnace operation.
Such deposits (which typically comprise non-pyroprocessed and/or partially or completely pyroprocessed material, ash and dust) can drastically limit the production capacity of the furnace and disable its stable operation.
Indeed, such deposits reduce the free internal cross area/diameter of the rotary furnace, which firstly creates a bottleneck for the material flow and also results in pressure drop increase over the length of the furnace. As a consequence of this pressure drop increase, when the fan, known as exhaust fan, supplying the secondary combustion oxidizer to the furnace via a downstream pyroprocessed material cooler, is operated at constant power, the amount of combustion oxidizer supplied to the furnace decreases, causing a decrease in the heat produced in the furnace and a corresponding decrease in pyroprocessed material production. If, alternatively, the power to the fan, which may be an Induced Draft or ID fan, is increased in order to overcome the increase in pressure drop and to maintain the level of combustion oxidizer supplied to the furnace, the energy efficiency of the production process is significantly reduced thereby.
Various methods have been proposed to limit ring formation and to remove those rings that have formed during furnace operation.
When ring formation is due to the recirculation in the kiln of a furnace atmosphere containing a large amount of impurities, such as sulfur or chlorine, a known basic solution is the use of a by-pass installation to extract part of the flue gases or of the furnace atmosphere, typically from 1 to 5%. This solution reduces global efficiency of the plant because heat from the flue gas and material present in the flue gas is lost with flue gas extraction. Moreover this solution is very complex to design and implies considerable additional capital cost.
Another curative solution consists in shooting out the ring by an industrial gun firing through the kiln hood, if the ring is not formed too far back from the kiln outlet. An alternative solution to mechanically break-up such deposits is to fire CO2 charges through the shell of the kiln at locations where ring formation occurs, provided ports are available thereto (see U.S. Pat. No. 2,301,855). U.S. Pat. No. 3,220,714 describes a further process for mechanically removing a material ring from a rotary kiln by cyclically applying vibratory mechanical energy to cause cracks in the ring and thereby to reduce the structural rigidity of ring. These known mechanical solutions can severely damage the (refractory material of the) kiln wall.
However, they do not reduce the process of ring formation in the furnace and require the furnace to be shut down and cooled before the rings can be removed.
In accordance with the process described in U.S. Pat. No. 4,421,563, solid fuel is first gasified, sulfur is removed from the produced gas and the cleaned gas is combusted in the rotary furnace. Such a process is capable of reducing those mechanisms of ring formation connected to the presence of ash and other combustion residues in the furnace. However, such a process is normally not of industrial interest as it increases the production costs of the hydraulic binder to inacceptable levels. Indeed, the reason why low quality fuels and other waste products are frequently burnt in the rotary furnace is exactly to keep production costs at a competitive low level.
In U.S. Pat. No. 5,882,190 a method of clinker production by burning high sulfur containing fuel is proposed whereby the sulfur content of the clinker is measured and whereby the oxygen content in the furnace flue gas is maintained sufficiently high so as to keep the temperature in the kiln below the decomposition temperature of CaSO4. The oxygen content in the flue gas is controlled by adjusting the speed of the exhauster (exhaust fan) sucking air trough kiln and tower and the plant. This solution is limited by the exhaust fan capacity and only addresses the problem of ring formation linked to excessive sulfur.
In FR-A-2246510, it is proposed to inject additional air at the inlet end (flue gas outlet) of a clinker kiln so as to lower the temperature of the atmosphere at the kiln inlet to a temperature lower than the temperature defining ring formation. This proposed solution drastically reduces the thermal efficiency of furnace and deals only with ring formation at the kiln inlet.
FR-A-2837916 proposes to change the flame length and the hot spot location by means of varying the oxidizer flow partition between two branched-off oxidizer injectors of a burner. Also described is an automatic flame-length control as a function of process needs such as the need to limit blockages. Associated with this method is a burner device consisting essentially of three concentric tubes with a fuel channel located between two oxidizer channels.
The practical implementation of the method as described in FR-A-2837918 is not suitable for pyro-processing in a rotary kiln as described above. Indeed, it is practically not possible to use the burner device described in FR-A-2837916 to use hot air from the material cooler, which typically has a temperature around 1100° C., to inject primary oxidizer and secondary oxidizer into the rotary kiln by branching off the secondary oxidizer from a common primary and secondary oxidizer supply line, as is the case according to FR-A-2837918, and to substantially vary the flow ratio between the two, which requires a suitable mechanical control device such as a valve and would entail a significant additional pressure drop which would have to be overcome by the exhaust fan, thereby reducing the profitability of the process.
For this reason, it is standard practice not to use hot cooling air for the primary oxidizer, but to supply the primary oxidizer injected through the burner assembly from a different oxidizer source. Using a common cold air source to generate the primary and secondary oxidizer by branching off is also not an option as this would represent a huge and unacceptable efficiency loss as typically about 20% of the total heat input into the rotary furnace is provided by the hot air from the material cooler.